The production of alkylene oxide, such as ethylene oxide, by the reaction of oxygen or oxygen-containing gases with ethylene in the presence of a silver-containing catalyst at elevated temperature is an old and well-known art. For example, U.S. Pat. No. 2,040,782, dated May 12, 1936, describes the manufacture of ethylene oxide by the reaction of oxygen with ethylene in the presence of silver catalysts which contain a class of metal-containing promoters. In Reissue U.S. Pat. 20,370, dated May 18, 1937, Leforte discloses that the formation of olefin oxides may be effected by causing olefins to combine directly with molecular oxygen in the presence of a silver catalyst. (An excellent discussion on ethylene oxide, including a detailed description of commonly used manufacturing process steps, is found in Kirk-Othmer's Encyclopedia of Chemical Technology, 4th Ed. (1994) Volume 9, pages 915 to 959).
The catalyst is the most important element in direct oxidation of ethylene to produce ethylene oxide. There are several well-known basic components of such catalyst: the active catalyst metal (generally silver as described above); a suitable support/carrier (for example alpha-alumina); and catalyst promoters, all of which can play a role in improving catalyst performance. Because of the importance of the catalyst in the production of ethylene oxide, much effort has been expended to improve catalyst's efficiency in producing ethylene oxide.
The use of zirconium and or silicon components as either promoters in the ethylene oxide catalyst or as modifiers to supports (that is carriers) used for such catalysts are also known.
U.S. Pat. No. 5,703,001 describes a rhenium-free silver catalyst promoted with an alkali metal component and a Group IVB component wherein the Group IVB component is added as a compound having a Group IVB cation. Soluble zirconium compounds where the Group IVB component is a cation are preferred.
U.S. Pat. No. 5,145,824 describes a rhenium-promoted ethylene oxide silver catalyst supported on a carrier comprising alpha alumina, an added alkaline earth metal in the form of an oxide, silicon in the form of an oxide, and from zero to about 10 percent (%) added zirconium in the form of the oxide. In U.S. Pat. No. 5,145,824, the term “oxide” is used to refer to simple oxides made up of only one metal as well as complex oxides made up of the indicated metal and one or more of the other metals. The amount of alkaline earth metal used in the carrier is from 0.05 to 4 weight percent (wt. %), measured as the oxide. Similarly, U.S. Pat. No. 5,801,259 describes an ethylene oxide catalyst comprising silver and promoters on a carrier prepared by mixing alpha alumina, alkaline earth metal oxide, silicon oxide, and from zero to about 15% of zirconium in the form of the oxide. The particle sizes of the ceramic components are chosen such that the packing density of the dried carrier precursor is not greater than that of the fired carrier; thereby eliminating the need for organic burnout agents. In '824 and '259 patents, the carrier mixture is formed from a starting mixture containing alpha-alumina, and requires the addition of alkaline earth metal oxide. The addition of the zirconium oxide component is optional.
There are several examples in the prior art of carriers used for ethylene oxide catalysts which contain silicon-containing compounds. U.S. Pat. No. 6,313,325 describes a method for the production of ethylene oxide wherein the carrier of the catalyst is obtained by adding an aluminum compound, a silicon compound and an alkali metal compound to a low-alkali content alpha-alumina powder. After calcination, this mixture is thought to provide a coating layer of alkali metal-containing amorphous silica alumina on the outer surface of the alpha-alumina carrier and the inner surface of the pores thereof. Canadian patent 1,300,586 describes a catalyst using a carrier composed mainly of alpha-alumina, silica, sodium, which has measurable acidity and crystals of Al6Si2O13 which are detectable by X-ray Diffraction analysis (XRD).
Several terms are commonly used to describe some of the parameters of catalytic systems for epoxidation of alkenes. For instance, “conversion” is defined as the molar percentage of alkene fed to the reactor which undergoes reaction. Of the total amount of alkene which is converted to a different chemical entity in a reaction process, the molar percentage which is converted to the corresponding alkylene epoxide, that is alkylene oxide, is known as the “efficiency” (which is synonymous with the “selectivity”) of that process. The product of the percent efficiency times the % conversion (divided by 100% to convert from %2 to %) is the percentage “yield”, that is, the molar percentage of the alkene fed that is converted into the corresponding epoxide.
The “activity” of a catalyst can be quantified in a number of ways, one being the mole percent of alkylene epoxide contained in the outlet stream of the reactor relative to that in the inlet stream (the mole percent of alkylene epoxide in the inlet stream is typically, but not necessarily, zero percent) while the reactor temperature is maintained substantially constant, and another being the temperature required to maintain a given rate of alkylene epoxide production. That is, in many instances, activity is measured over a period of time in terms of the molar percent of alkylene epoxide produced at a specified constant temperature. Alternatively, activity may be measured as a function of the temperature required to sustain production of a specified constant mole percent of alkylene epoxide. The useful life of a reaction system is the length of time that reactants can be passed through the reaction system during which results are obtained which are considered by the operator to be acceptable in light of all relevant factors.
Deactivation, as used herein, refers to a permanent loss of activity and/or efficiency, that is, a decrease in activity and/or efficiency which cannot be recovered. As noted above, production of alkylene epoxide product can be increased by raising the temperature, but the need to operate at a higher temperature to maintain a particular rate of production is representative of activity deactivation. Activity and/or efficiency deactivation tends to proceed more rapidly when higher reactor temperatures are employed. The “stability” of a catalyst is inversely proportional to the rate of deactivation, that is, the rate of decrease of efficiency and/or activity. Lower rates of decline of efficiency and/or activity are generally desirable.
To be considered satisfactory, a catalyst must have acceptable activity and efficiency, and the catalyst must also have sufficient stability, so that it will have a sufficiently long useful life. When the efficiency and/or activity of a catalyst has declined to an unacceptably low level, typically the reactor must be shut down and partially dismantled to remove the catalyst. This results in losses in time, productivity and materials, for example silver catalytic material and alumina carrier. In addition, the catalyst must be replaced and the silver salvaged or, where possible, regenerated. Even when a catalyst is capable of regeneration in situ, generally production must be halted for some period of time. At best, replacement or regeneration of catalyst requires additional losses in production time to treat the catalyst and, at worst, requires replacement of the catalyst with the associated costs. It is therefore highly desirable to find ways to lengthen the useful life of a catalyst.